How to use singly linked lists and circular linked lists in JavaScript

function LinkedList(){ 
 /*节点定义*/ 
 var Node = function(element){ 
  this.element = element; //存放节点内容 
  this.next = null; //指针 
 } 
 
 var length = 0, //存放链表长度 
  head = null; //头指针 
 
 this.append = function(element){ 
  var node = new Node(element), 
   current; //操作所用指针 
 
  if (!head){ 
   head = node; 
  }else { 
   current = head; 
 
   while(current.next){ 
    current = current.next; 
   } 
 
   current.next = node; 
  } 
 
  length++; 
  return true; 
 }; 
 
 this.insert = function(position, element){ 
  if (position >= 0 && position <= length) { 
   var node = new Node(element), 
    current = head, 
    previous, 
    index = 0; 
 
   if(position === 0){ 
    node.next = current; 
    head = node; 
   }else{ 
    while(index++ < position){ 
     previous = current; 
     current = current.next; 
    } 
    node.next = current; 
    previous.next = node; 
   } 
 
   length++; 
   return true; 
  }else{ 
   return false; 
  } 
  }; 
 
 this.removeAt = function(position){ 
  if(position > -1 && position < length){ 
   var current = head, 
    previous, 
    index = 0; 
 
   if (position === 0) { 
 
    head = current.next; 
 
   }else{ 
 
    while (index++ < position){ 
     previous = current; 
     current = current.next; 
    } 
 
    previous.next = current.next; 
   }; 
 
   length--; 
   return current.element; 
  }else{ 
   return null; 
  } 
 }; 
 
 this.remove = function(element){ 
  var current = head, 
   previous; 
 
  if(element === current.element){ 
   head = current.next; 
   length--; 
   return true; 
  } 
  previous = current; 
  current = current.next; 
 
  while(current){ 
   if(element === current.element){ 
    previous.next = current.next; 
    length--; 
    return true; 
   }else{ 
    previous = current; 
    current = current.next; 
   } 
  } 
  return false; 
 }; 
 
 this.remove = function(){ 
  if(length < 1){ 
   return false; 
  } 
 
  var current = head, 
  previous; 
 
  if(length == 1){ 
   head = null; 
   length--; 
   return current.element; 
  } 
 
  
  while(current.next !== null){ 
   previous = current; 
   current = current.next; 
  } 
 
  previous.next = null; 
  length--; 
  return current.element; 
 }; 
 
 this.indexOf = function(element){ 
  var current = head, 
   index = 0; 
 
  while(current){ 
   if(element === current.element){ 
    return index; 
   } 
   index++; 
   current = current.next; 
  } 
 
  return false; 
 }; 
 
 this.isEmpty = function(){ 
  return length === 0; 
 }; 
 
 this.size = function(){ 
  return length; 
 }; 
 
 this.toString = function(){ 
  var current = head, 
   string = &#39;&#39;; 
 
  while(current){ 
   string += current.element; 
   current = current.next; 
  } 
  return string; 
 };  
 
 this.getHead = function(){ 
  return head; 
 } 
  
}

function CircularLinkedList(){ 
 var Node = function(element){ 
  this.element = element; 
  this.next = null; 
 } 
 
 var length = 0, 
  head = null; 
 
 this.append = function(element){ 
  var node = new Node(element), 
   current; 
 
  if (!head) { 
   head = node; 
   node.next = head; 
  }else{ 
   current = head; 
 
   while(current.next !== head){ 
    current = current.next; 
   } 
 
   current.next = node; 
   node.next = head; 
  }; 
 
  length++; 
  return true; 
 }; 
 
 this.insert = function(position, element){ 
  if(position > -1 && position < length){ 
   var node = new Node(element), 
    index = 0, 
    current = head, 
    previous; 
 
 
   if (position === 0) { 
 
    node.next = head; 
    head = node; 
 
   }else{ 
 
    while(index++ < position){ 
     previous = current; 
     current = current.next; 
    } 
 
    previous.next = node; 
    node.next = current; 
 
   }; 
 
   length++; 
   return true; 
  }else{ 
   return false; 
  } 
 }; 
 
 this.removeAt = function(position){ 
  if(position > -1 && position < length){ 
   var current = head, 
    previous, 
    index = 0; 
 
   if (position === 0) { 
 
    head = current.next; 
 
   }else{ 
 
    while (index++ < position){ 
     previous = current; 
     current = current.next; 
    } 
 
    previous.next = current.next; 
   }; 
 
   length--; 
   return current.element; 
  }else{ 
   return null; 
  } 
 }; 
 
 this.remove = function (element){ 
  var current = head, 
   previous, 
   indexCheck = 0; 
 
  while(current && indexCheck < length){ 
   if(current.element === element){ 
    if(indexCheck == 0){ 
     head = current.next; 
     length--; 
     return true; 
    }else{ 
     previous.next = current.next; 
     length--; 
     return true; 
    } 
   }else{ 
    previous = current; 
    current = current.next; 
    indexCheck++; 
   } 
  } 
  return false; 
 }; 
 
 this.remove = function(){ 
  if(length === 0){ 
   return false; 
  } 
 
  var current = head, 
   previous, 
   indexCheck = 0; 
 
  if(length === 1){ 
   head = null; 
   length--; 
   return current.element; 
  } 
 
  while(indexCheck++ < length){ 
   previous = current; 
   current = current.next; 
  } 
  previous.next = head; 
  length--; 
  return current.element; 
 }; 
 
 this.indexOf = function(element){ 
  var current = head, 
   index = 0; 
 
  while(current && index < length){ 
   if(current.element === element){ 
    return index; 
   }else{ 
    index++; 
    current = current.next; 
   } 
  } 
  return false; 
 }; 
 
 
 this.isEmpty = function(){ 
  return length === 0; 
 }; 
 
 this.size = function(){ 
  return length; 
 }; 
 
 this.toString = function(){ 
  var current = head, 
   string = &#39;&#39;, 
   indexCheck = 0; 
 
  while(current && indexCheck < length){ 
   string += current.element; 
   current = current.next; 
   indexCheck++; 
  } 
 
  return string; 
 };  
 
}